The present invention relates to a system for transmitting the power of an engine to four wheels of a four-wheel drive vehicle and more particularly to a system provided with a viscous coupling for transmitting the power.
A four-wheel drive vehicle of the type in which a transfer clutch in a form of a dog clutch, hydraulic clutch or electromagnetic clutch is provided for transmitting the output torque of a transmission to auxiliary drive wheels is known. In such a vehicle, a power transmission system can be converted from a two-wheel drive system to a four-wheel drive system by engaging the transfer clutch. However, if a dog clutch is employed as a transfer clutch, the difference of speeds between front wheels and rear wheels cannot be absorbed when the clutch is engaged. Although a hydraulic clutch slips to absorb the speed difference, a complicated hydraulic control system must be installed.
Recently, a viscous coupling containing a high viscous fluid such as a silicon oil has been proposed as a transfer clutch. The viscous coupling operates to transmit the torque in proportion to the difference between input speed and output speed. The viscous coupling may either be used as a devise for locking a control differential, or be used as a transfer clutch which operates to transfer the torque to the auxiliary wheels to provide a four-wheel drive system when the auxiliary wheels skid. Moreover, the slipping in the viscous coupling prevents a tight corner braking at cornering.
Japanese Utility Model Application Laid Open No. 59-188731 discloses a power transmitting system for a four-wheel drive vehicle provided with a viscous coupling. A four-wheel driving mode or a two-wheel driving mode can be selected by connecting and disconnecting the transfer device. However, in the four-wheel driving mode, power is transmitted to the rear wheels while the viscous coupling slips so that the power transmitting characteristic and driveability of the vehicle decrease and the fuel consumption increases. Additionally, since temperature of the silicon oil contained in the viscous coupling rises as the result of the continuous slipping, the viscosity of the oil decreases. Thus, clutch torque becomes too small for increasing driveability on muddy and snowy rough roads.
In order to eliminate such drawbacks, it is preferable to lock the viscous coupling so as to directly connect the front and rear drive systems with each other. However, if the viscous coupling is adapted to be manually locked, the locking of the coupling is determined by the decision of a driver of the vehicle, which is disadvantageous to beginners.